So, you wanna do practice questions? That's great! I'm glad you're actively engaging with the material and trying to do better.
However, there's a pitfall to doing practice questions that I want you to be aware of.
The way I started doing 61A practise tests was quite straightforward. Print the exam, do it best I could, then check answers for where there were mistakes. Try not to make those mistakes again. Rinse and repeat.
The problem with this approach was that it was passive engagement with the material. CS isn't like AP History, where you can learn all the mistakes and never make them again. That's because you literally can't make the same mistake again –– every question is unique.
Each mistake in 61A has a reason: an incomplete way of thinking, confusion about a concept, misreading a question, or even laziness. Without the reason, you'll keep making the same mistakes over and over again.
What's worse –– you won't even realize you're doing this. You won't see the commonality behind your mistakes because you won't be looking for any.
Now that I've highlighted a potential problem, you may come up with your own way to fix or avoid it. I'll share how I changed my approach.
I started making comments all over the paper. Things like return type? string and single line: use λ helped me think in a streamlined manner and improved the way I approached questions. More importantly, it created a reference for me to go back to when checking my answers.
Then, I made comments with a different color, this time about what was incorrect, not only in the answer but also my thinking. As a simplified example, the incorrect answer may be 17 instead of 18, but the actual problem was forgetting to apply PEMDAS.
This seems obvious, but gets harder to do as the difficulty of the question increases. When both your answer and the exam solution seem equally complex, it's hard to tell why you're wrong and they're right. You must not give in to the urge to shrug and move on. Don't give up on a question till you understand exactly what you did wrong and the official solution did right.